Rotational relaxation in gases and its spectral manifestations

Abstract

The rotational energy relaxation and the angular momentum relaxation of linear molecules are described in the framework of the nonadiabatic external field approximation (EFA) in which translational motion is considered classically. The collision operator calculated in this approximation is corrected to satisfy simultaneously the requirement of both detailed balance and S-matrix unitarity. After this correction, the correlation functions of rotational energy and angular momentum have been calculated in the high-temperature limit. It is proved that in this limit they are monoexponential. The corresponding energy (τ E ) and angular momentum (τ J ) relaxation times are expressed through the transition probabilities of the rotational levels and the latter through collision phases. The relation τ J =2τ E , peculiar for nonadiabatic, but weak rotational relaxation, is confirmed. This proves that RT relaxation at high temperature corresponds to the so-called correlated J-diffusion process, which is realized in the J-space by small steps. It is shown that the information about the time τ E is contained in the Raman spectral wings and, after collapse, may be extracted from the line width narrowing.